LONDON -- Iran's diplomatic mission to Britain partly reopened its doors Thursday for the first time in two years as part of a wider agreement on the thawing of tensions between the countries.

Diplomats in London said that only the Iranian embassy's consular section, which handles visas and commercial affairs, resumed operations Thursday, while its political section remained closed. Nonetheless, Britain said the move represented another welcome sign of progress.

"This is the next stage of the step-by-step process of taking forward our bilateral relationship with Iran," Britain's Foreign Office said in a statement.

Iran made its London move as part of an agreement with Britain -- announced Thursday on the sideline of wider negotiations in Vienna between Iran and six world powers -- to end both countries' reliance on formal "protecting power" arrangements. Under that system in force since 2011, Sweden's embassy in Tehran had handled the consular needs of British citizens in Iran, while Oman did the same for Iran in London.

"We will no longer have protecting powers acting as intermediaries. Diplomatic contact will now be made direct between our two countries," the Foreign Office said. It emphasized that British and Iranian officials would talk directly to each other "through non-resident charges d'affaires and officials." A charge d'affaires is typically the second-highest diplomatic official below the ambassador.

As part of their agreement, Britain removed the Swedish flag and an accompanying Swedish "British interests" sign from the British embassy building in Tehran. In its place a British embassy plaque and an "Embassy closed" sign were erected. The Foreign Office said, in practice, any British citizens living in Iran who needed consular assistance would still have to use other EU-member embassies operating in Tehran.

Earlier Thursday, media reports billed the Iranian embassy in London as fully reopened.

However, despite the presence of a newly hoisted red, white and green flag outside Iran's west London embassy building -- and the apparent presence of Iranian staffers inside -- the formal reopening of both countries' political sections, and return of ambassadors, awaits further negotiations.

The British Embassy in Tehran was closed in late 2011 after hard-liners overran the building. Iran's embassy in London also was closed. Relations between London and Tehran have improved since moderate Iranian President Hassan Rouhani rose to power in August.